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ATEL # 1026; D.Steeghs (CfA), J.Kennea (PSU), M.A.P.Torres (CfA), J.Miller (Michigan), J.Homan (MIT), J.Greiner (MPE), G.Sala (MPE)
on 11 Mar 2007; 2:22 UT
Password Certification: Danny Steeghs (firstname.lastname@example.org)
Subjects: X-ray, Binaries, Black Holes, Transients
We report on Swift TOO observations of the black-hole transient XTE J1856+053 in response to its recent activity (ATel #1024).
The Swift observations lasting 1300s started on March 10th at 20:40UT with the XRT configured in photon counting mode. A bright source with a flux consistent with the recently reported activity level of XTE J1856+053 was observed in the field of view at a position of R.A.(J2000)=18:56:42.86, Dec(J2000)=+05:18:33.8 with a 90% uncertainty radius of 3.5". This position is within the 1.8' error circle that was derived for XTE J1856+053 during a previous outburst (Marshall et al. 1996, IAU Circ. No. 6504).
Since the high count-rate resulted in considerable pile-up, we excluded the central (6 pixel) portion of the PSF and extracted an XRT spectrum using the pile-up free wings only. An absorbed diskbb model fit to the spectrum provided a reduced Chi^2 of 117/100 with an absorbing column of N_H=2.6 +/- 0.1 10^22 cm^-2 and a T_in = 0.90 +/- 0.03 keV. For comparison, an absorbed powerlaw fit gave Chi^2=101/100 with N_H=4.4 +/- 0.2 10^22 cm^-2 and a photon index of 3.9 +/- 0.1. A marginally better fit was achieved with a brems model giving Chi^2=94/100 for N_H=3.2 +/- 0.2 10^22 cm^-2 and kT = 1.64 +/- 0.09. For comparison, the galactic column towards XTE J1856+053 is estimated to be 3.3 10^22 according to the dust maps of Schlegel et al. (1998).